In bimodal FM-AFM, two flexural modes are excited simultaneously. The totalvertical oscillation deflection range of the tip is the sum of the peak-to-peakamplitudes of both flexural modes (sum amplitude). We show atomically resolvedimages of KBr(100) in ambient conditions in bimodal AFM that display a strongcorrelation between image quality and sum amplitude. When the sum amplitudebecomes larger than about 200 pm, the signal-to-noise ratio (SNR) isdrastically decreased. We propose this is caused by the temporary presence ofone or more water layers in the tip-sample gap. These water layers screen theshort range interaction and must be displaced with each oscillation cycle.Further decreasing the sum amplitude, however, causes a decrease in SNR.Therefore, the highest SNR in ambient conditions is achieved when the sumamplitude is slightly less than the thickness of the primary hydration layer.
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